Materials having a permittivity of at least 20 are important for use in capacitors for advanced DRAMs such as the 1 Gbit generation and beyond. However, it has not been easy to maximize the overall storage charge of capacitors containing high dielectric constant materials. Events that occur during the fabrication process can reduce the final overall charge storage capability of the capacitor.
Capacitor performance can be increased by reducing the capacitor leakage current. Leakage current is a stray current which flows across the surface of a high dielectric constant capacitor or alternatively through the capacitor.
Leakage currents can cause unpredictable and detrimental changes in circuit conditions. It is therefore advantageous to reduce leakage currents in circuits. Thermal cycling is one way to reduce leakage currents. In thermal cycling, the high dielectric constant capacitor is heated to a critical temperature either before or after the overlying metallic layer is applied. Thermal cycling can also lead to increased adherence of the overlying metallic layer.
Additionally, new high dielectric constant material are being actively sought for the next generation of DRAM capacitors. Perovskites are an important class of high dielectric constant materials. Perovskites can be ferroelectrics and have a crystalline structure. Members of the perovskite family include BaTiO.sub.3, SrTiO.sub.3, LiNbO.sub.3 and (Ba,Sr)TiO.sub.3. (Ba,Sr)TiO.sub.3 is a titanate that contains a mixture of barium, Ba, and strontium, Sr, and is also known as BST.
Overall storage capacity can be effected in different ways. Interfacial layers between the high dielectric layer and the electrodes can reduce the effective capacitive potential of a high dielectric constant material by providing a leakage path. There are a number of ways that a detrimental interfacial layer could be formed. The interfacial layer could be produced by a reaction of a previously deposited electrode with the process chemistry during the deposition of the high dielectric layer. Alternatively, the interfacial layer could be formed by a residual layer left at the surface of the high dielectric layer after its deposition or the layer could be a single monolayer or less of contaminants formed on the surface of the high dielectric layer.
The control of capacitance across an interfacial area between two electrodes is important because the interfacial area contributes to series capacitance. The total capacitance of the area between the two electrodes is given by the equation C=.epsilon..sub.0 .epsilon..sub.r A/d; where .epsilon..sub.0 is the permittivity of free space, .epsilon..sub.r is the relative permittivity of the dielectric material, A is the area and d is the separation between electrodes. When an interfacial area consists of more than one layer, the different layers all contribute to the overall capacitance.
When the dielectric constants of the layers comprising the interfacial area between two electrodes are not equal, each layer contributes to the determination of .epsilon..sub.r, the relative permittivity of the high dielectric constant material. The variable .epsilon..sub.r is related to the relative permittivities and thicknesses of the layers between the two electrodes and is represented by the equation: 1/.epsilon..sub.r /d=1/.epsilon..sub.r1 /d.sub.1 +1/.epsilon..sub.r2 /d.sub.2. Therefore it is desirable to maximize the dielectric constant for each layer since each layer contributes to the overall dielectric constant of the interfacial area.
One way to reduce interfacial interference at a surface is to clean the surface prior to electrode introduction. Many different methods exist to clean semi conductor component materials. Wet and dry pre-electrode deposition cleaning methods are known in the art but tend to be material specific. While pre-electrode cleaning solutions and methods do exist for other materials, there is no teaching of a method of increasing the overall performance of a high dielectric constant material that substantially modifies only the surface of a constituent material.
Thus there remains a need for improved performance of a high dielectric constant capacitive materials such that the overall performance of the material is maximized, the surface layer of a constituent material is affected, and the leakage current is reduced.